1. Field of the Invention
The invention relates in general to a digital analog signal converting apparatus, and more particularly, to a gamma correction apparatus for a liquid crystal display.
2. Description of the Related Art
Featuring the favorable advantages of thinness, lightness, and low electromagnetic radiation, liquid crystal displays (LCDs) have been widely used nowadays.
An LCD panel includes a plurality of pixels arranged in matrix form. Each pixel is composed of an upper plate, a lower plate and a liquid crystal layer set between the lower plate and the upper plate. When the potential difference between the upper plate voltage and the lower plate voltage changes, the liquid crystal molecular arrangement of the liquid crystal layer will change accordingly. As a result, the pixel luminance is affected. Therefore, the luminance of the pixels of an LCD can be controlled by adjusting the magnitude of voltages applied to the lower plate and the upper plate respectively. The difference between the upper plate voltage and the lower plate voltage is called the “gray-scale voltage”.
Please refer to FIG. 1, a diagram of gamma curve illustrating the relationship between pixel voltage and pixel luminance. As illustrated in FIG. 1, the relationship between pixel voltage and pixel luminance is nonlinear. In addition, pixel luminance is related to the magnitude of pixel voltage but is not related to the polarity of pixel voltage. The gamma curve, thus, is symmetrical to the Y-axis, with a positive polarity gamma curve 102 and a negative polarity gamma 104 on the both sides of the Y-axis. According to the gamma curve, when pixel voltages of the same magnitude are individually applied to a pixel, the pixel will generate the same level of luminance regardless of the polarity of the pixel voltage. If it is desired to display a pixel at the same luminance over a long period of time, the liquid crystal molecules can be protected by alternating the polarity of the pixel voltage applied to the pixel.
Normally, pixel signals are binary digital signals. Since the gamma curve relationship between pixel voltage and pixel luminance is non-linear, the LCD needs a particular circuit to convert digital pixel signals into corresponding pixel voltages according to the gamma curve relationship and to output the pixel voltages to achieve a linear relationship between pixel signal and pixel luminance. This conversion is called “gamma correction”, which is used to improve the display quality of an LCD panel.
Please refer to FIG. 2, a schematic diagram illustrating the theory of gamma correction. When executing gamma correction, first of all, a plurality of pixel signals are selected as reference pixel signals. In FIG. 2, pixel signals D0, D1, D2, D3, and D4 are selected as reference pixel signals. According to the gamma curve, each reference pixel signal corresponds to a positive polarity reference voltage and a negative polarity reference voltage respectively. Take pixel signal D0 for example; D0 corresponds to positive polarity reference voltage V0 and negative polarity reference voltage V9 respectively. By the same analogy, reference pixel signals D0, D1, D2, D3, and D4 respectively correspond to five positive polarity reference voltages V0, V1, V2, V3, and V4, and five negative polarity reference voltages V9, V8, V7, V6, and V5 as shown in FIG. 2. During gamma correction, the corresponding pixel voltages of other pixel signals can be obtained via interpolation based on the relationship between the reference pixel signals and reference voltages. Each pixel corresponds to a positive polarity pixel voltage and a negative pixel voltage respectively.
It is noteworthy that the more pixel signals are selected for gamma correction, the more accurate the corresponding pixel voltage of each pixel signal estimated will be. Normally 8 pixel signals are selected for the execution of gamma correction. According to the gamma curve, 8 pixel signals correspond to 8 positive polarity reference voltages and 8 negative reference voltages respectively. The gamma correction device thus executes gamma correction based on the 16 reference voltages.
Please refer to FIG. 3, a schematic diagram for a conventional gray-scale voltage generating circuit. Normally, pixel signals, denoted by DATA, are signals of 8-bit binary data which can represent at most 256 gray levels. Therefore, a gray-scale voltage generating circuit 300 needs to be set in the gamma correction device to output 256 positive polarity gray-scale voltages and 256 negative polarity gray-scale voltages according to inputted reference voltages, wherein each gray-scale voltage corresponds to a pixel signal DATA. gray-scale voltage generating circuit 300 is composed of two series of resistors for outputting positive polarity gray-scale voltages and negative gray-scale voltages respectively. Each series of resistors have 255 resistors, numbered as R0, R1, . . . , R254; a plurality of input nodes for the input of corresponding reference voltage signals V0 to V4 and V5 to V9; and 256 output nodes for outputting gray-scale voltages. According to voltage dividing rule, by setting appropriate resistance value of each resistor in the two series of resistors, the corresponding gray-scale voltage of each of the digital pixel signals DATA can be outputted from each output nodes in the two series of resistors.
Please refer to FIGS. 4A to 4G, diagrams illustrating gamma curves of various patterns. To make the diagrams simpler and clearer, FIGS. 4A to 4G illustrate only part of the gamma curve with the remaining part of corresponding complete gamma curves left to be inferred from the illustrated part in FIGS. 4A to 4G. According to what color is to be displayed, three kinds of pixel signals corresponding to the red, green and blue colors are employed to control the luminance of the red, green and blue pixels respectively in a color LCD. In FIGS. 4A to 4G, the three gamma curves labeled R, G and B represent the gamma curve relationship between the luminance of a pixel and the gray-scale voltage of the pixel when the pixel is used to display the red, green, and blue colors respectively. The gamma curve relationship between the gray-scale voltage applied to the pixel and pixel luminance changes when the conformation of the pixel's liquid crystal molecules changes. The possible patterns of the gamma curve are illustrated in FIGS. 4A to 4G.
In FIG. 4A, as the pixel voltages applied to pixels of different colors approach their maxima, the luminance difference between the pixels of different colors turns larger. FIG. 4B shows that as the pixel voltages applied to pixels of different colors approach their minima, luminance difference between the pixels of different colors turns larger. FIG. 4C, a mixture of FIGS. 4A and 4B, shows that whatever the pixel voltages applied to pixels approach their maxima or minima, luminance difference between the pixels of different colors turns larger. FIG. 4D is similar to FIG. 4A except that when the pixel voltage applied to the pixel reaches its maximum, the liquid crystal molecules will have the same light transmittance no matter what color the pixel displays. FIG. 4E is similar to FIG. 4B except that when the pixel voltage applied to a pixel reaches its minimum, the light transmittance of the liquid crystal molecules will be the same no matter what color the pixel displays. FIG. 4F is similar to FIG. 4C except that when the pixel voltage applied to a pixel reaches whatever its maximum or minimum, the light transmittance of the liquid crystal molecules will be the same no matter what color the pixel displays. FIG. 4G shows that the luminance difference between pixels of different colors turns smaller as the pixel voltages applied to the pixels approach their minima or maxima, but turns larger as the pixel voltages are getting closer to middle values. The gamma curve of an ordinary TN mode LCD panel is basically the same as the gamma curve shown in FIG. 4A, while the gamma curve of an ordinary VA mode LCD panel is basically the same as the gamma curve shown in FIG. 4B.
It can be understood from FIGS. 4A to 4G that patterns of gamma curve vary with the conformation of the liquid crystal molecules in an LCD panel. However, they share one common characteristic: the gamma curve changes when the displaying color of the pixel changes.
When executing gamma correction, the conventional gamma correction device determines the relationship between the pixel signal and the reference voltage according to an already established gamma curve disregarding what color the corresponding pixel of each pixel signal displays, thereby determining the magnitude of the corresponding pixel voltage of each pixel signal. This method avoids the circuit of the gamma correction device becoming too complicated and prevents the drive circuit of the gamma correction device from occupying too large a space. However, the conventional method is disadvantaged by failing to execute gamma correction of pixel signals with respect to the colors of the pixels to which the pixel signals are to be applied. In this way, a linear relationship between the pixel signal and the pixel luminance, as well as the maximum luminance, under certain circumstances, can not be obtained. Therefore, the display quality of the LCD panel is affected.